[Gdal-dev] Re: Gdal and NetCDF...

[Emilio Mayorga]

Oi Daniel!

Good to see you here. I don't have an answer for you (I've never used
the GDAL bindings for netcdf, although I do have a lot of experience
with netcdf), but I found this recent thread in the FWTools list that
refers to exactly the same problem. The poster was using OpenEV to
access a netcdf file, but OpenEV must use the same GDAL code to access
netcdf files.
http://lists.maptools.org/pipermail/fwtools/2006-February/000279.html

Maybe Frank has looked into the problem more closely, recently?

Good luck,

-Emilio


> From: "Daniel Victoria" <daniel.victoria at gmail.com>
> To: gdal-dev at lists.maptools.org
> Date: Tue, 21 Mar 2006 15:10:24 -0300
> Subject: [Gdal-dev] Gdal and NetCDF...
> Hi all,
>
> I have a very simple python program that subsets and converts the CRU
> 0.5 degrees time series data in a NetCDF format following COARDS
> conventions - hopefully -
>
> Now, I want to convert this NetCDF to TIFF using gdal and import it to
> GRASS. When I ask my program to subset the CRU data from 10 to 30
> degrees latitude it will get 41 cells from 10.25 to 30.25 (I'm using
> cell centers). This means that the data spans from 10 (south edge) to
> 30.5 (north edge), with 41 cells, which equals 0.5 degrees resolution.
>
> Now, when I do a gdalinfo on the NetCDF file I get a resolution of
> 0.4878 (equals 20 degrees / 41 cells).
>
> Could someone point me to what I'm doing wrong? Is this a feature/bug?
> I'm using GDAL provided by the FWTools-linux-1.0.0a7
>
> I'm attaching the information on the NetCDF file (ncdump and gdalinfo)
>
> Thanks a bunch
> Daniel
>
> ----- ncdump output -----
>
> netcdf saida {
> dimensions:
>         X = 41 ;
>         Y = 41 ;
>         T = 12 ;
> variables:
>         double Y(Y) ;
>                 Y:long_name = "Latitude" ;
>                 Y:units = "degrees_north" ;
>         double X(X) ;
>                 X:long_name = "Longitude" ;
>                 X:units = "degrees_east" ;
>         short T(T) ;
>                 T:long_name = "Months" ;
>                 T:units = "months since 1981-01-01" ;
>         short pre(T, Y, X) ;
>                 pre:long_name = "Precipitation" ;
>                 pre:units = "mm" ;
>                 pre:scale_factor = 0.1 ;
>                 pre:missing_value = -9990 ;
>
> // global attributes:
>                 :Conventions = "COARDS" ;
>                 :history = "Tyndall Centre grim file created on 13.01.2004 at 14
> :28 by Dr. Tim Mitchel" ;
>                 :production = "CRU TS 2.1 converted to NetCDF by Daniel" ;
> data:
>
>  Y = 30.25, 29.75, 29.25, 28.75, 28.25, 27.75, 27.25, 26.75, 26.25, 25.75,
>     25.25, 24.75, 24.25, 23.75, 23.25, 22.75, 22.25, 21.75, 21.25, 20.75,
>     20.25, 19.75, 19.25, 18.75, 18.25, 17.75, 17.25, 16.75, 16.25, 15.75,
>     15.25, 14.75, 14.25, 13.75, 13.25, 12.75, 12.25, 11.75, 11.25, 10.75,
>     10.25 ;
>
>  X = 75.25, 75.75, 76.25, 76.75, 77.25, 77.75, 78.25, 78.75, 79.25, 79.75,
>     80.25, 80.75, 81.25, 81.75, 82.25, 82.75, 83.25, 83.75, 84.25, 84.75,
>     85.25, 85.75, 86.25, 86.75, 87.25, 87.75, 88.25, 88.75, 89.25, 89.75,
>     90.25, 90.75, 91.25, 91.75, 92.25, 92.75, 93.25, 93.75, 94.25, 94.75,
>     95.25 ;
>
>  T = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ;
>
> <snip> file values...
>
>
> ------ GDAL info output --------
> Driver: netCDF/network Common Data Format
> Size is 41, 41
> Coordinate System is `'
> Origin = (75.250000,30.250000)
> Pixel Size = (0.48780488,-0.48780488)
> Metadata:
>   NC_GLOBAL#Conventions=COARDS
>   NC_GLOBAL#history=Tyndall Centre grim file created on 13.01.2004 at 14:28 by D
> r. Tim Mitchel
>   NC_GLOBAL#production=CRU TS 2.1 converted to NetCDF by Daniel
>   pre#long_name=Precipitation
>   pre#units=mm
>   pre#scale_factor=0.1
>   pre#missing_value=-9990
>   X#long_name=Longitude
>   X#units=degrees_east
>   Y#long_name=Latitude
>   Y#units=degrees_north
>   T#long_name=Months
>   T#units=months since 1981-01-01
> Corner Coordinates:
> Upper Left  (  75.2500000,  30.2500000)
> Lower Left  (  75.2500000,  10.2500000)
> Upper Right (  95.2500000,  30.2500000)
> Lower Right (  95.2500000,  10.2500000)
> Center      (  85.2500000,  20.2500000)
>
> <snip> band information...